Article by Glenn Kohler, DNR Forest Entomologist
This spring and summer, the Washington State Department of Natural Resources (DNR) received numerous reports of dead or damaged Douglas-fir trees throughout the state. Symptoms include entirely red crowns in saplings and red tops or scattered red branches in trees. The damage is more common in dry lowland areas and sites with well-drained soils.
DNR forest health specialists examined Douglas-firs with these symptoms and found unexpected levels of attack by several species of bark beetles such as Douglas-fir engraver, (Scolytus unispinosus), Douglas-fir pole beetle (Pseudohylesinus nebulosus), and another engraver beetle (Scolytus monticolae which has no common name.
These beetle species are normally considered ‘‘secondary’ because they typically infest trees that are first weakened by a larger, primary issue such as root disease, fire damage, or drought stress. Secondary beetles can damage trees under stress but normally lack the capacity to kill live, healthy trees.
Douglas-firs have been particularly affected by the hotter- and drier-than-normal conditions we’ve experienced in Washington over the past three summers. Back-to-back years of drought is stunting the health of Douglas-firs, leaving them less able to fend off insect attacks.
These conditions have allowed secondary bark beetle species to establish in healthy Douglas-firs, and boost beetles populations, causing significant damage or even death in some cases. Secondary bark beetles mostly prefer to attack small-diameter trees, yet recent investigations have found them in stems of larger diameter trees as well, which is historically uncommon.
Consider the following to prevent or manage secondary beetle attacks in Douglas-fir:
- Keep forested stands thinned so that remaining trees can access more of the water stored in soils
- Irrigate and mulch high value yard or park trees during prolonged periods of drought
- Avoid fertilizing trees as this can increase foliage growth and the need for more water
- Do your homework before applying pesticides:
- Some systemic pesticides can be applied as a preventative measure for high value trees
- Pesticides are not effective in controlling beetles in trees that have already been attacked
- Consider hiring a licensed pesticide applicator to ensure the proper selection, timing and application of pesticides
Unfortunately, pheromone treatments, such as those used to deter the more aggressive Douglas-fir bark beetle (Dendroctonus pseudotsugae), are not available to combat these minor bark beetle species.